The present disclosure relates to an LED backlight device capable of turning on a plurality of light emitting diodes used as a light source of a liquid crystal display device.
Typically, liquid crystal display devices are lightweight, thin, and consume low-power. The liquid crystal display device includes a light source such as a Light Emitting Diode (LED). Strings of LEDs have been implemented and driven by various current driving circuits.
For example, FIG. 13A is a block diagram schematically illustrating a conventional LED backlight device and FIG. 13B is a circuit diagram of a convention current driving circuit of FIG. 13A. LED backlight device 100 includes a DC power supply 101, a DC converter 102, a bottom detecting circuit 103, LED strings 104, 105, 106, 107, and current driving circuits 108, 109, 110, 111.
The DC converter 102 converts a DC voltage supplied from the DC power supply 101 into a DC voltage needed to switch on the LED strings 104, 105, 106, 107 to provide the converted DC voltage to the LED strings 104, 105, 106, 107, respectively. The current driving circuit 108, as illustrated in FIG. 13B, includes an error amplifier 108a, a Field Effect Transistor (FET) 108b, and a resistor R. The current driving circuits 109, 110, 111 may be configured to be substantially the same as that in FIG. 13B. With the current driving circuit 108, current flowing into a source electrode of the FET 108b from a corresponding LED string 104 is detected by the resistor R, and the error amplifier 108a compares the detected voltage with a reference voltage REF to control a voltage being supplied to a gate electrode of the FET 108b. The current flowing via the LED string 104 can be constantly controlled by controlling a gate voltage of the FET 108b. As described above, the gate voltage of the FET 108b can be controlled such that the current constantly flows via the resistor R and a drain electrode of the FET 108b. 
Returning to FIG. 13A, the bottom detecting circuit 103 detects one having a minimum value from among voltages of respective lowermost cathodes of the LED strings 104, 105, 106, 107, and outputs a minimum value detection signal to the DC converter 102. Forward voltages of the LED strings 104, 105, 106, 107 may be different from one another due to a difference between elements. For this reason, although currents flowing via the LED strings 104, 105, 106, 107 are constantly controlled by the current driving circuits 108, 109, 110, 111, respectively, forward voltages of the LED strings 104, 105, 106, 107 may be different from one another. The DC converter 102 may need to supply a DC voltage greater than a maximum forward voltage among the forward voltages of the LED strings 104, 105, 106, 107. A difference between LED forward voltages can arise due to a difference between peripheral temperatures of the LED strings 104, 105, 106, 107. The bottom detecting circuit 103 may force DC voltages supplied to the LED strings 104, 105, 106, 107 to be adjusted to a required minimum value by detecting one, having a minimum value, from among voltages of respective lowermost cathodes of the LED strings 104, 105, 106, 107 and outputting a minimum value detection signal to the DC converter 102. That is, the DC converter 102 may adjust DC voltages being supplied to the LED strings 104, 105, 106, 107 such that a voltage level of the minimum value detection signal provided from the bottom detecting circuit 103 becomes constant.
In addition to the above-described driving circuit, a driving circuit for an LED backlight is disclosed in JP Laid-open No. 2007-208113. With an LED driving device disclosed in JP Laid-open No. 2007-208113, low-power driving is made by detecting a terminal voltage of each serial LED group and controlling the terminal voltage so as to become a power supply voltage needed for the whole.
An LED driving device disclosed in JP Laid-open No. 2009-54998 uses a low current control manner in which a driving voltage is supplied to an LED group via a transformer, a current flowing upon switching-on of the LED group is detected, and the detected current is controlled to have a predetermined current value.
A light emitting diode switching device disclosed in JP Domestic re-publication of PCT international application No. 2007-69371 is configured such that a current is alternately output to a serial LED group from a plurality of boosting circuits being connected in parallel, the serial LED group is grounded via a resistor, and a voltage across the resistor is fed back so as to be constant.
An LED driving circuit disclosed in JP Laid-open No. 2006-319221 has a shunt coil formed by connecting one end of each of two coils via a tap and two rectifier diodes each respectively connected to one end and the other end of the shunt coil, and respective serial LED groups are connected to two rectifier diodes. With this configuration, increase in a temperature or a life difference due to an irregular quantity of light and different current values is suppressed.
In the above-described LED backlight device 100, the current driving circuits 108 through 111 may be a constant current circuit which is configured to constantly control the current flowing at the drain electrode of the FET 108b. Power loss of the LED backlight device 100 may be determined by the product of an output voltage and a constant current of the constant current circuit. If the output voltage of the constant current circuit becomes high, power loss may increase. That is, heat may be generated. If the difference between forward voltages of the LED strings 104, 105, 106, 107 becomes large, a DC voltage supplied from the DC converter 102 may increase excessively, and output voltages of the current driving circuits 108, 109, 110, 111 may increase, generating heat. This problem may not be solved via devices disclosed in the above-described references. Further, the LED backlight device 100 may necessitate the bottom detecting circuit 103 as a feedback circuit. Devices disclosed in the above-described references may necessitate a feedback circuit for detecting a current or a terminal voltage of an LED group. For this reason, a control system may be complicated, and it may be troublesome to adjust circuit parameters. A need therefore exists for a less-complicated, heat minimizing approach to driving strings of LEDs in an LCD backlight device.